Generic methods
- Dart course
- Intermediate
- Lesson 31/116
- 5 min
Learn to define generic methods.
Generic methods
A method can also take its own type parameter. The type parameter is written before the return type, inside angle brackets.
Example: T first<T>(List<T> items) { return items[0]; }. Here the first method works on any list; it returns the first element. first<int>([1, 2, 3]) returns an int, first<String>(['a']) returns a String. The type parameter belongs to the method; the class does not need to be generic. This is the clean way to write type-safe helper methods.
Type inference at the call site
When calling a generic method you can write the type argument explicitly or let Dart infer it. first<int>([1, 2, 3]) is explicit; first([1, 2, 3]) is inferred.
During inference Dart looks at the argument type and decides T from it. Since [1, 2, 3] is a List<int>, T becomes int. The result is still type safe; the compiler catches wrong usage. Explicit writing shows the type to the reader at once; inference keeps the code short. Both forms give the same compile-time safety.
Practice Write and call a first<T> method
Define a generic method named T first<T>(List<T> items); its body should be return items[0];. In main, call it with var n = first<int>([10, 20, 30]); and print(n).
Starter code
// Define the first<T> method
void main() {
// Call first<int>([10, 20, 30]) and print
}Sample solution
// Define the first<T> method
T first<T>(List<T> items) {
return items[0];
}
void main() {
var n = first<int>([10, 20, 30]);
print(n);
}Expected output
10
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Add a type parameter to first
T first____(List<T> items) {
return items[0];
}T first<T>(List<T> items) {
return items[0];
}Explanation: A generic method is written as T first<T>(List<T> items).
2 What does first<int>([10, 20, 30]) return?
- The last element of the list
- The length of the list
- The first element of the list Correct answer
- The whole list
Explanation: The first method returns the first element of the list.
3 Match the generic method parts
| Term | Meaning |
|---|---|
T |
Return type |
first |
Method name |
List<T> |
Parameter type |